The primary immunologic abnormality resulting from infection by human immunodeficiency virus (HIV) is the progressive depletion and functional impairment of T lymphocytes expressing the CD4 cell surface glycoprotein. The loss of CD4 helper/inducer T cell function probably underlies the profound defects in cellular and humoral immunity leading to the opportunistic infections and malignancies characteristic of the acquired immunodeficiency syndrome (AIDS) (Lane et al., Ann. Rev. Immunol. 3:477, 1985). Studies of HIV-1 infection of fractionated CD4 and CD8 T cells from normal donors and AIDS patients have revealed that depletion of CD4 T cells results from the ability of HIV-1 to selectively infect, replicate in, and ultimately destroy this T lymphocyte subset (Klatzmann et al., Science 225:59, 1984). The possibility that CD4 itself is an essential component of the cellular receptor for HIV-1 was first indicated by the observation that monoclonal antibodies directed against CD4 block HIV-1 infection and syncytia induction (Dalgleish et al., Nature 312:767, 1984; McDougal et al., J. Immunol. 135:3151, 1985). This hypothesis has been confirmed by the demonstration that a molecular complex forms between CD4 and the major envelope glycoprotein of HIV-1 (McDougal et al., Science 231:382, 1986)
The major envelope protein of HIV-1 is a glycoprotein of approximately 160 kD (160). During infection proteases of the host cell cleave gp160 into gp120 and gp41. gp41 is an integral membrane protein, while gp120 protrudes from the mature virus. Together gp120 and gp41 make up the HIV envelope spike.
The HIV envelope spike mediates binding to receptors and virus entry (Wyatt and Sodroski, Science 280:188, 1998). The spike is trimeric and composed of three gp120 exterior and three gp41 transmembrane envelope glycoproteins. CD4 binding to gp120 in the spike induces conformational changes that allow binding to a coreceptor, either CCR5 or CXCR4, which is required for viral entry (Dalgleish et al., Nature 312:763, 1984; Sattentau and Moore, J. Exp. Med. 174:407, 1991; Feng at al., Science 272:872, 1996; Wu et al., Nature 384:179, 1996; Trkola et al., Nature 384:184, 1996).
The mature gp120 glycoprotein is approximately 470-490 amino acids long depending on the HIV strain of origin. N-linked glycosylation at approximately 20-25 sites makes up nearly half of the mass of the molecule. Sequence analysis shows that the polypeptide is composed of five conserved regions (C1-C5) and five regions of high variability (V1-V5).
With the number of individuals infected by HIV-1 approaching 1% of the world's population, an effective vaccine is urgently needed. An enveloped virus, HIV-1 hides from humoral recognition behind a protective lipid bilayer. An available viral target for neutralizing antibodies is the envelope spike. Genetic, immunologic and structural studies of the HIV-1 envelope glycoproteins have revealed extraordinary diversity as well as multiple overlapping mechanisms of humoral evasion, including self-masquerading glycan, immunodominant variable loops, and conformational masking. These evolutionarily honed bathers of diversity and evasion have confounded traditional means of vaccine development. It is believed that immunization with effectively immunogenic HIV gp120 envelope glycoprotein can elicit a neutralizing response directed against gp120, and thus HIV. The need exists for immunogens that are capable of eliciting an immunogenic response in a suitable subject. In order to be effective, the antibodies raised must be capable of neutralizing a broad range of HIV strains and subtypes.